Ask a physicist

[wiseguy]

During a rousing game of ball ladder the other day, I was answering some questions shmo had in preparation for his upcoming physics test. It gave me the idea for this thread.

Rules:
1) Don't ask me to just solve things for you. If you don't understand something, I'll try to help you figure it out. I ain't doing your homework, so don't try.

2) Ask questions with enough time to get an answer! I know the last minute is when all good work happens, but particularly on a forum you're just going to be screwed if you don't give us time to respond.

3) Please stay on topic! Follow-up questions are great, but try to keep this thread useful.

4) Remember, there is no substitute for asking your teacher/professor/TA! They know what you're working on and how they want you to approach the problem, so don't fool yourself into thinking some guy on a cartoon airplane forum is going to solve all your troubles.

If you know your physics and are qualified* to help explain something, feel free.

*I know there are smart guys and girls out there not done with school yet, but for quality assurance I'd prefer if the helpers have a degree in the field (or are close to getting one)...I think there are enough of us out there to make this possible.

So, any questions?

[sunshineduck]

a town barber shaves all and only those men in town who do not shave themselves

who shaves the barber?

[darknietzsche]

I just graduated with my bachelors degree in Chemical Engineering and going in the fall to start on my PhD. I would also be willing to help in this area if allowed. I have good physic's expertise, but also am willing to help with chemistry and math (through differential equations) and of course classes directly related to chemical engineering if anyone needs it.

Thanks Wiseguy for the thread. And I totally agree with the rules you have put into effect.

[elxir]

Good one offer your services after school ends for the summer

i see through your ruse wiseguy

[Jrathje]

Why wouldn't this work?



And this one. What's wrong with it?



Those are my questions. I look forward to your answers. Thank you.

[Mt.Vesuvius]

Edit: wtf u post editor

[VeRiTaS]

Originally Posted by Jrathje

Why wouldn't this work?

get back the magnet thingy !

[Jrathje]

Ok, I promise THIS is actually my last question. Is this a good idea?

[Ribilla]

Studying First year general engineering at Cambridge, I would offer my help, but I doubt you'll get anything other than troll posts.

I do have a question though, which came up as part of a logic design question where you had to design a logic to combine two 2-bit numbers A1, A0 and B1, B0 in the form N=A^B, with outputs N4, N3, N2, N1, N0 using JK bistables

The logic is easy to work out, and we were told to assume that x^0=1 for all x. My quesiton is, what is the proof for 0^0 = 1?

The only thing I can think of is a binomal expansion in the form

(1+x)^n = 1 + nx + O(n^2)

By putting x = -1 and n = 0 you get

0^0 = 1 + 0 +0 +0.... = 1 + n*0 = 1.

However, I am not a mathematician, does this actually work?

EDIT: For instance, is the result only due to the fact that binomial series is defined with 0^0 =1?

[Urpee]

Originally Posted by Ribilla

My quesiton is, what is the proof for 0^0 = 1?

Are you sure the exercise asks you to proof that 0^0 = 1. It rather sounds to me that they suggest to define it to be that. That's a hint for the answer by the way.

[Duck Duck Pwn]

anything to the 0 power is 1 by definition; you don't prove the assumption

[Ribilla]

I have already done the question, I just want to know why it is defined as such.

[tyr]

I was wondering about something for the last couple of days :
If you build a really tall building that goes all the way from ground to space (let's not talk about the feasibility of the thing) and therefore solidly anchored to the ground, how would air react in the parts of the building when it's starting to become scarce. Would it be exactly the same as outside ?
What would then happen if you start compartmentalizing it and setting the pressure to the normal ground-level air pressure (~1 bar I believe), would it remain the same throughout the whole compartment (still at the higher parts of the building) ?
I thought about the space station example but the difference here is that it's not exactly in orbit as it's anchored to the ground so I was wondering how gazes would react to the gravity in that case.

[malkin]

Speaking of buildings, here is my question (which I already know the answer to). But, "Do you think the World Trade Center towers fell the way they were supposed to?"

[Ribilla]

Originally Posted by tyr

I was wondering about something for the last couple of days :
If you build a really tall building that goes all the way from ground to space (let's not talk about the feasibility of the thing) and therefore solidly anchored to the ground, how would air react in the parts of the building when it's starting to become scarce. Would it be exactly the same as outside ?
What would then happen if you start compartmentalizing it and setting the pressure to the normal ground-level air pressure (~1 bar I believe), would it remain the same throughout the whole compartment (still at the higher parts of the building) ?
I thought about the space station example but the difference here is that it's not exactly in orbit as it's anchored to the ground so I was wondering how gazes would react to the gravity in that case.

Ignoring all the obvious reasons why this would no be possible:

If you made one huge compartment and set the lowest level at atmospheric pressure than the pressure any point in your tower would be the same as the pressure outside of the tower. This is because pressure gradients are due to two things. Hydrostatic distribution and acceleration normal to the streamline direction. Since the air in your tower isn't moving particularly fast (we assume) there is no pressure gradient due to acceleration. This means the pressure is purely due to the weight of the fluid above it. The weight of air (in a vertical column centred over a small element of air in your tower) is exactly the same as the weight of air centred over a small element outside your tower, so the pressures are equal

If you made lots of little compartments that were completely sealed, the pressure would be whatever it was in the compartment when you sealed it. If they are not sealed, it normalises back to hydrostatic over the whole tower.

The reason for this is as follows:

Generally any system will do whatever it takes to minimise it's free energy (move further down the energy 'well') So lets imagine you built your tower with sea level atmospheric pressure all the way through, and it was completely sealed. You then opened a door at the bottom of the tower what happens?

There will be a large pressure gradient near the top of your tower between the Patm of your tower and the ~vaccume of space. This creates a lot of very large forces, in laymen's terms, your system doesn't 'like' big forces they create strain energy in any material, so it will do whatever it can to get rid of that stored energy. This either means rupturing or loweing the pressure. When you open the door air will rush out, releiving the pressure at the top of your tower.

You can also think of a forces arguement, take a general element cube of air with side length dx. The pressure above is P and the pressure below is P + dP. The cube has mass m = qv (q = density). Therefore there is a vertical force of dx^2 * P (F = Pressure times area) pushing down on the cube, a vertical force of dx^2 * (P+dP) acting up and another vertical force due to weight of mg = q*dx^3*g

From the forces balancing we can see that dP = gqdx

Or dp/dx = gq. I.e. the infinitesimal change in pressure over an infinitesimal vertical distance is density times the gravitational acceleration. This is the pressure gradient so we know that pressure changes linearly over distance where g remains constant. At high altitudes g will be much less, so the gradient will be less. Factors like temperature also effect density and this derivation assumes incompressibility.

I.e. because each tiny cube of air has a weight which needs to be balanced, the pressure on top must be less than the pressure on the bottom.

I hope this helps, I wasn't that clear on what you were actually asking. In a space station, there is nowhere for the air to go, if it is sealed, so the pressure normalises across the whole station and the material just has to cope with large stresses.

[tyr]

I knew pretty much everything you said but I found an acceptable answer to my original question in there. ^^
There was something I forgot about all this and you reminded it to me so it's all good.
Thanks for the answer.

[Urpee]

Originally Posted by Ribilla

I have already done the question, I just want to know why it is defined as such.

Strictly 0^0 cannot be one number because you can set up analytic equations that if you approach 0^0 will give different limits.

However it turns out that an analytic continuation of x^0=1 is much more often important than 0^x. Taking the power of 0 is a rather trivial operation that almost never means much at all (except in the field of resolving multiple singularities, but that's a highly specialized area of math).

Hence why many mathematicians have promoted that 0^0=1 is the right definition in almost all cases.

To get a sense of the history and some of the positions on this point see:

http://www.cs.uwaterloo.ca/~alopez-o...xt/node14.html

[MajorPayne257]

****ing magnets, how do they work?

[Urpee]

The magnetic force, roughly speaking, is the force created by parallel moving electric charges. The most direct example is two parallel wires which you have currents flowing in the same direction. They will move towards each other. If the currents go in opposite direction they will repel.

Magnets have trapped circular paths inside them that are highly aligned. If you bring another magnet close, it is very much like the wire. If the orientation is the same it will attract, else repel.

The current understanding of this is even deeper, because special relativity tells us that this is actually just a specific case of general electro-magnetic interactions. Charges who move in parallel reference frames have different forces associated with them than those that frames that move away from each other.

Why electromagnetic forces exist at all, noone currently knows, but it's not what the ICP tries to sell

[Kuja900]

In the recent Xmen movie, Magneto lifts a submarine with his power of magnetism. He does this while in an aircraft. Now because every force has an equal and opposite reaction shouldn't Magneto be weighed down equivalent to the weight of the submarine therefore bringing down the aircraft?

[Urpee]

All physics depicted in super-hero stories is absolutely accurate *nods*

[Kuja900]

Originally Posted by Urpee

All physics depicted in super-hero stories is absolutely accurate *nods*

My question was that if this movie was to be physically accurate is that not what should have happened.

[Urpee]

Well assume that rather than magneto being the source of the magnetic force, he had a loop of current that creates the force. What are the consequences of using it? Yes he would be creating attractive forces to metals before and behind him. So if the wall behind him is such that it sufficiently pulls him back while he is also pulled forth he may well be able to stand static, but it depends very much on the geometry and the ferromagnetic details of the metal.

And to your actual point, in principle it's a mass equation. The lighter object will move more. But it is complicated by friction forces, the submarine being subject to water friction, and the airplane being subject to airflow/bernoulli forces. Instinctively, also given gravity, it'd expect virtually nothing to happen to the submarine, but the plane being pulled down.

So yeah it's fiction.

[Knipchip]

Assuming there isn't an intelligent being up there who willingly created a place in which the laws of physics were defined in such a way that something can come into being that asks this question, it isn't an outcome of extreme chance and that only in a place that resembles a place with physics as we know it life can originate, could there be an infinite amount of 'big bangs', until finally one big bang coincidentally developed into a universe in which a thing that asks such a question could originate? So are there multiple universes, ones beyond ours? Would these universes resemble ours, or would they be unimaginably different to ours, like having totally different physics our brains could in no way grasp, such as not consisting of spatial dimensions? Wondering whether anyone has a speculation in the right direction.

[nobodyhome]

Usually I can figure out troll physics pictures pretty quickly but this one seems to have me stumped. How does this one not work?

[Tekn0]

Originally Posted by Knipchip

Assuming there isn't an intelligent being up there who willingly created a place in which the laws of physics were defined in such a way that something can come into being that asks this question, it isn't an outcome of extreme chance and that only in a place that resembles a place with physics as we know it life can originate, could there be an infinite amount of 'big bangs', until finally one big bang coincidentally developed into a universe in which a thing that asks such a question could originate? So are there multiple universes, ones beyond ours? Would these universes resemble ours, or would they be unimaginably different to ours, like having totally different physics our brains could in no way grasp, such as not consisting of spatial dimensions? Wondering whether anyone has a speculation in the right direction.

String theory already suggests this that there might be an infinite number of meta-stable "strings", which can create millions of universes just like ours with 3 dimensions and time, but with varying degrees of particles and forces. What is also interesting is the possibility of universe creation and destruction are mere phases in an infinite cycle and that the big bang was not the beginning but the end of the previous one.

[Ribilla]

Originally Posted by nobodyhome

Usually I can figure out troll physics pictures pretty quickly but this one seems to have me stumped. How does this one not work?

Either the magnetic force is stronger or the gravitational force is stronger, depending on your magnet.

If gravity is stronger, nothing happens.

If the force from the magnet is stronger, the fluid will simply stay stuck to the magnet and won't fall.

[Void]

Originally Posted by nobodyhome

Usually I can figure out troll physics pictures pretty quickly but this one seems to have me stumped. How does this one not work?

Have you played "Feed Me Oil" on Iphone ?

[IMG]

[Urpee]

Is there any ferromagnetic liquid that is adhesive not aversive? I certainly don't know of any. But aversive liquids are pushes downward not upward by the capillary effect.

[MajorPayne257]

Currently there are experiments going on that, through quantum nonlocality, attempt to send 'messages' into the past via entangled photons. Now we're of course only talking about a fraction of a second before, but the idea is that the second photon will receive the 'message' before the first photon even sends it.

How plausible is this and what do your great minds think about it?
Or has it already happened? I heard about this awhile ago.

[Selfish Lover]

[JDR]

If I cum in my girlfriend's hair, which brand of shampoo should she use to best remove it 6 hours later?

[Mt.Vesuvius]

False.

The next poster has gypsy magic.

[[Y]]

False.

This community isn't ready to handle a physics discussion thread maturely.

[JDR]

List all forces present in this picture:

[Ribilla]

Originally Posted by JDR

List all forces present in this picture:

Obesity and repulsion.

Obesity wins, that's why they so saggy.

[wiseguy]

This is why we can't have nice things, guys....man I need to be a mod.

However, regarding photons reacting to things that haven't happened yet, look up "The Amazing Mandel Experiment." Guaranteed to blow your mind. Here's the short(ish) version...please look up terms as necessary, I can't explain everything in this one:

Now, laser light is coherent, so when waves from two beams in the same location interfere with one another. Peaks in the light waves add, so where there are two peaks you get a bright spot, when one peak and one trough meet you get a dark spot. Look up "superposition" for more on this. When you point a laser pointer at a wall you can see this on a disordered scale--little bright and dark flecks in the reflection. Interference at work! If you have them nicely set up, those big laser beams will make pretty diffraction patterns when they interfere with each other on a wall, like vertical bands of light and dark. With me so far? Good!

First it helps to understand the two slit experiment. For this, one laser beam is split through two slits in a sheet of paper, and you get two beams coming out of those slits which can interfere on a wall, and make that interference pattern.



If you cover one slit, however, there is no second beam to interfere and there is no interference pattern on the other side. This also holds true if you send photons through one at a time--there is only one photon reaching both slits, but as long as it could pick EITHER one it goes through BOTH! This is fundamentally trippy and should not feel right in your brain. "But it can't!" Your brain says. But oh, it does, says physics...hold on, it's going to get bumpy. If you get information about which slit the single photon chooses (by covering one), you break the superposition of states (left and right simultaneously) and kill the interference. "Still, you're just covering one, it can't really be because you found out the quantum secret, right?"

Some nuts and bolts before we go deeper. There are these gizmos called nonlinear crystals. High energy (blue) light goes in, two lower energy (green and red) light beams come out. This also works for single photons of light (very small wave packets) rather than continuous beams. Just an optics tool, don't worry about how exactly that happens.

So you take two of these crystals and set them next to one another. Now you take your blue laser and split it in half with a semi-silvered mirror. Half the beam is aimed at one crystal, half the beam at the other. Two beams go in, four total come out (two red and two green)...so far so good, right?

Now, back to the Amazing Mandel Experiment. The red beams are lined up such that they go to detectors that blip "left" or "right" depending on which crystal (left or right) they came from. The green beams are pointed at a screen so they can interfere. When a continuous beam of blue light is sent into the crystals, both L and R red beams go "tick" in the detectors and the green beams interfere, as you expect.

The weird part is when one photon goes through--only L or R will "click" because the one photon has to pick a crystal, and since you found out which crystal (which slit the green beams are coming from), no interference happened. You spoiled the surprise, and you don't get interference. On the other hand, if you make the red beams overlap so you CAN'T tell which crystal was the source of green photons, you get the interference patten. Okay, still sounds like a double slit experiment....but wait. Almost there!

You can make the beam pathway lengths different, so that the green beams will interfere or NOT interfere close after the crystal, but red light won't be detected or not detected until much later. This could be seconds later, but--and experiment showed this to be true--the green photons KNOW if you will detect the red photons at any point in the future. If you're going to find out which crystal was the source, the green ones don't give you a pattern. If you NEVER find out, there's interference.

The green photons KNOW what is going to happen to their red buddies. Okay, they don't have brains, but the future is directly influencing the present! Or the present is influencing the past, however you want to look at it.

That's why the experiment is Amazing

[dr. carbon]

Hey wiseguy,

In line with the ability for the green photons to know the future for the red, I was reading about quantum consciousness or consciousness at the quantum level, the other day, and I had another similar example.

According to the Uncertainty Principle, it is impossible to know the exact position and momentum of a particle at a given moment, which is shown through the equation: delta x * delta p(momentum) >/= reduced plank's constant/2. Furthermore, we also know that when one observes something, say an electron, you actually influence what is being observed as seen in the Copenhagen Interpretation where observing something collapses its wave function.

However, when you entangle two subatomic particles, particles A and B, and you "let them loose," you can measure the position of one and the momentum of the other. Since the particles are entangled, you can calculate the position and momentum of the other particles (I don't know exactly how except for the fact that it's because they are entangled and have opposite spins). Now, through this calculation, you have acquired both the momentum and the position of the subatomic particles, which contradicts the Uncertainty Principle.

So evidently, consciousness or some other force seems to exist in subatomic particles. Also interesting is that if you change the spin of particle A (entangled with B), the spin change of particle B is instantaneous, faster than the speed of light. So in this case, the particles are conscious of each other without time as a factor.

If you think of a dimension as a way a body or state can change (something can change in the 1st dimension, 2nd dimension, 3-dimensionally, and through time ("chronitude"), one can also notice that something can change instantaneously through it's consciousness of some other active body or state, and many scientists have started to consider consciousness as the 5th dimension.

[dr. carbon]

And in your interference diagram, are the black and white areas visual representations of the nodal and anti-nodal lines that occur from two-source interference?

[dr. carbon]

Originally Posted by Knipchip

Assuming there isn't an intelligent being up there who willingly created a place in which the laws of physics were defined in such a way that something can come into being that asks this question, it isn't an outcome of extreme chance and that only in a place that resembles a place with physics as we know it life can originate, could there be an infinite amount of 'big bangs', until finally one big bang coincidentally developed into a universe in which a thing that asks such a question could originate? So are there multiple universes, ones beyond ours? Would these universes resemble ours, or would they be unimaginably different to ours, like having totally different physics our brains could in no way grasp, such as not consisting of spatial dimensions? Wondering whether anyone has a speculation in the right direction.

I have created most of this theory based on a few assumptions, and I and my mentors will soon submit a scientific paper on it when I am of age.)

This one is based off the quote, "We do what we must, because we can."

Although applying this to human actions makes the reasoning seem kind of circular, as you are essentially saying we do what we can because we can, and since must falls in the same category as can, we do what we must because we can.

However, if you apply this to existence, you see a whole new world of possibilities, bringing me to my second point that God is entropy. I will re-apply the quote to everything at the end.

Entropy is the tendency for a body to increase in disorder through the changing of useable energy into unuseable energy, which is the reason energy is always lost into some useless state disallowing us from creating a perpetual machine. Furthermore, it is also noted that this growing disorder in the system is responsible for seeming randomness, and that disorder or delta s always rises

If you were to take a boulder teetering of a cliff, and you leave it completely undisturbed, the probability of the boulder falling is 0. However, if you take this state through an infinite time period, the boulder will fall because the disorder within the system is enough to make the boulder's position change, so you can get to the understanding that:

event with probability 0 [over] an infinite time period = event with probability 1.

Now if you say that the universe does have a start, it is in a nonchanging state. In this beginning, absolutely nothing happens. Except for entropy. Eventually this entropy causes the disorder in the system to increase, and over an infinite time period, every permutation of chance becomes possible. Various laws and forces of physics can be treated as a permutation of chance with entropy as the divine nature that started it all. Now as the disorder keeps increasing, our universe keeps expanding. However, something is wrong:

When you take an ordered deck of cards and shuffle it till you reach maximum disorder (maximum unuseable energy in the case of the universe), every shuffle from then on will decrease the disorder of the deck and useable energy in our universe. So at a certain point of maximum expansion, our universe will become smaller, as the probability for the next shuffle to decrease disorder is greater than it increasing disorder. At a certain point as it becomes smaller, the probability for disorder increasing and decreasing becomes even, and through an infinite time period, both will happen. Because of this, the expansion and recession of the universe may or may not reach its max or minimum. But because there IS A CHANCE that it could, over an infinite time period, IT WILL HAPPEN, so you will encounter infinite big bangs over the infinite time period.


Specific Explanation:
As of now, we humans always assume that something must have a start and an end. However, we find it impossible to perceive that something just is. The universe never had a beginning. Instead, it is simply an infinite cycle. When an expanded state, black holes reverse the expansion until all matter is combined into a single point. Eventually, because you can calculate the exact position of this point since it is so small, the "internal momentum" of the points "innards" becomes so unstable which causes the matter to rapidly expand, which we call as the big bang. Eventually this expansion is counteracted by black holes and the process repeats. With this philosophy, there is no need to question the beginning of the laws of physics or the universe, because there IS NO BEGINNING, but they simply just are.




Now back to the quote. According to the multi-verse theory, there is one universe for every permutation of chance of happening. In one universe, I will die tomorrow, in this one, I hopefully will not, and so on. If you consider the existence of various physical laws and forces as a permutation of chance, it is possible that other universes have whacked out forces and laws. The only reason we live in a world where things are the way they are is because we live in a universe of whose permutation of chance just happens to be what it is. The only reason things happen is because the CAN happen, and over an infinite time period, as proven, anything that CAN happen, WILL happen.